When alternating current at elevated frequencies passes through a wire, a phenomenon occurs that forces the outer surface or skin of a wire to carry most of the electrical current. This phenomenon, called the skin effect, increases in intensity as the frequency of operation increases. One type of wire that compensates for the skin effect is Litz wire. Litz wire, from the German “Litzendraht”—Litz for “strands” of wire and Draht for “wire”, is a type of electrical conductor that comprises multiple strands of individually insulated wires that allow the flow of high frequency alternating electrical current without appreciable loss because of electrical resistance or impedance in the conductors. The diameter of individual Litz wire strands is very small, and its radius is about the same or less than the depth of skin effect conduction. By bundling many individually insulated small wires together, the skin effect may be negated.
In addition to Litz wire, bundles of ordinary stranded wire employed at low frequency are not appreciably affected by the Skin Effect. As a result, stranded wire may be employed in electric motors, transformers, etc. Stranded wire overcomes many problems encountered with a single solid wire having about the same total diameter. If the stator of an electric motor stator is wound with a large diameter wire or a heavy cable is placed into the lamination of a transformer, it would be difficult to bend the wires into place. However, many individually insulated smaller diameter wires may be substituted for one large insulated solid wire. A bundle of smaller diameter wires having an overall diameter of a single wire may be bent and angled much more easily than the single large diameter wire.
Since Litz wire is made of many individually insulated wires, it is difficult to terminate unless all of the insulation in the termination area is first removed. Unfortunately, removing this insulation may damage the wire's physical structure. Until recently, low temperature film insulation was used, which was removed by dipping the insulated wire bundle into molten solder to burn off the insulation. However, the trend has been to use high temperature film insulation, which molten solder cannot remove. Crimp terminals cannot make a satisfactory connection, because the terminal's insulation piercing teeth cannot reach every strand in the Litz wire bundle. Another method is to use fused salts to chemically dissolve the organic film insulation. This does not work well in production because the chemical residue from the salts cannot be completely removed from inside the Litz wire bundle after the insulation is dissolved. Removing the insulation strand by strand is a labor intensive and time consuming process.
To date, the only proven successful production method to terminate large bundles of insulated wire is tube fusing, using the SN-Fusing process. The general term “fusing” is a method of joining low resistance metals with a type of machine similar to a resistance welding machine, but without appreciable distortion or damage to the parts being joined. Normally, when copper is resistance welded (i.e., spot welded, butt welded, etc.), it is drastically distorted to a point where some of the metal looses its physical integrity. This does not occur with fusing.
There are different methods of wire fusing, including “commutator fusing,” also known as “tang fusing,” and “hot staking” or “SN-fusing.” In commutator/tang fusing, the parts are heated, cleaned, softened, and pushed together until all the air between them is eliminated, and the high points of one metal part are pushed into the low points of the other. A surface adhesion contact holds the parts together.
With SN-fusing, or tin-fusing, a diffusion metallurgical bond is developed. If tin is heated until it is liquid, and a bar of copper is inserted into the molten liquid tin, the copper bar eventually dissolves. This solvent action, called wetting, is what permits tin to coat copper by dissolving its surface molecules. Tin adheres the surface of copper with a strength comparable to that which a piece of solid metal holds itself together, that is, by the attraction between adjacent atoms. Tin, being attached by such attractive forces, cannot be mechanically pried from the surface of the copper. Further, tin cannot be completely drained or wiped away when molten or liquid, since the surface of the copper remains permanently wetted by a film of the tin. A copper/tin inter-metallic compound is formed whenever tin wets copper. This compound itself is not strong. Therefore, by using a minimum amount of tin brought in contact with a base copper alloy for as short a time as possible, the copper may be “tinned” (i.e., wetted or coated), while keeping the basic strengths and properties of copper. For tinning to occur, the copper must be relatively clean of any foreign matter.
SN-fusing comprises six basic steps as illustrated in FIG. 1. In Step S1, fusing pressure is applied to the parts that are being joined, until a preset level of pressure is reached. In step S2, heat is applied to a fusing electrode(s), and then dissipated into the parts being fused. In Step S3, as heat in the parts being fused increases, the wire's film insulation, if any, is vaporized. In Step S4, the tin, which is at its molten point, acts as a solvent to clean the surface of the copper conductors. In step S5, as the dissipated heat increases, the tin and any inter-metallic compounds are vaporized and/or driven from the joint's interface. In step S6, the resultant ultra-pure copper at the interface of the parts is then fused, resulting in a diffusion bond. However, the fusing pressure must be continually applied until the joint cools to a reasonable temperature, otherwise the plastic metals may separate because of the contractual forces that are being applied to them during cooling.
When many individual wire leads need to be joined together or to other stranded or solid wire, a tinned tube may be used. The tube acts as a gathering device as well as a mechanical terminal. The wires are placed into a tinned copper tube. Fusing electrodes then engulf the tube and it is fused. The advantage of the tube is that the tin inside the tube cannot easily be removed from the joint area during heating, since the tube holds it in place. Therefore, the tin wets most (or all) of the wires inside the tube before it is driven out of the joint. This means that most, if not all, of the conductors inside the tube, will be cleaned by the tin.
Unfortunately, wetting all of the copper wires with tin does not always result because the amount of tin that coats the terminal's tube is extremely small. To overcome this problem, additional heat and compression may be applied. The tin and the compression sealing of the joint help protect the termination from outside environmental pollution. This pollution is normally atmospheric gases as well as moisture. However, when the termination, after fusing, is immersed in a liquid that has a very thin consistency, the liquid may be drawn into the termination through capillary action, where the non-tin coated material is not diffusion bonded to the wires in the center of the bundle, or at the ends of the tube where the wires enter the tube. Most liquids will not damage a diffusion bonded joint if the wires keep their tin coating just outside of the joint area. Certain fluids, such as human body fluids, tend to corrode the wires at the point where they enter the termination's tube structure. This corrosion weakens the physical integrity of the wire bundle as it enters either side of the tube. Eventually, the wire bundle may be weakened to a point where the wires actually break or they have lost enough strands so that the current passing through them overheats because of resulting high resistance area(s) in the wire. As this over-heated area continues to heat the wire, the electrical connection may be destroyed.
Further, while fusing without tin is normally employed to join bare stranded wire to other bare stranded wire or solid bare wire, in certain circumstances, tin coated tubes may also be used. The phenomenon of moisture entering the terminating tube may also occur when tin is used to coat the interior of the tube.
If this type of corrosion and erosion continues for some time, the electrical characteristics of the termination may be in jeopardy because of the resulting increase in the electrical resistance, which may affect the wire's ability to conduct electrical power. This has happened to pacemaker or defibrillator leads that are inserted into a human body for prolonged periods of time. At the point of lead failure, the only solution is to replace the leads. Of course, this means that a surgical procedure needs to be performed.
One method of solving the corrosion problem is to apply a hot melt plastic material to both ends of the tube. However, this may not be easily done automatically. Also, for an intimate seal to result between the metal terminal and the plastic material, both materials would have to reach the temperature of, or above, the melted plastic. If the entire terminal's ends do not reach this temperature, the plastic will not make intimate contact with the metal.
Accordingly, what would be desirable, but has not yet been provided, is a system and method for insulating fused Litz or bare stranded wire terminations that overcomes the deficiencies in the prior art described hereinabove.